Tagged scale inhibitor compositions and methods of inhibiting scale

ABSTRACT

Scale inhibitor compositions and methods of inhibiting scale formation generally include a tagged scale inhibiting (co)polymer including at least one scale inhibiting moiety and an imidazole moiety. The imidazole moiety fluoresces at a wavelength of about 424 nm and can be used to detect the amount of scale inhibitor present.

BACKGROUND

The present disclosure generally relates to tagged scale inhibitioncompositions and methods of inhibiting scale. More particularly, thepresent invention relates to imidazole tagged scale polymeric inhibitorsfor use in water treatment and/or oil field applications.

Scale inhibiting polymers are often used in water treatment and oilfield applications to minimize and/or prevent scale deposition. Thedeposition of scale can occur in the transport of aqueous mixtures andin subterranean rock formations due to the presence of water bearingalkaline earth metal cations such as calcium, barium, strontium, and thelike as well as the presence of anions such as phosphate, sulfates,carbonates, silicates and the like. When these ions are in sufficientconcentrations, a precipitate can form that builds up on interiorsurfaces of the conduits used for transport or in the subterranean rockformations, which restrict flow of the media of interest, e.g., water oroil. In oilfield applications, scales that are commonly formed includecalcium sulfate, barium sulfate, and/or calcium carbonate that aregenerally formed in the fresh waters or brines used in well stimulationand the like as a result of increased concentrations of these particularions, the water pH, pressures, and temperatures. In addition, calciumphosphate can form from the phosphate chemistry that is commonly used totreat wells and pipes for corrosion. The buildup of these mineralprecipitates can reduce or block flow in the conduits and rockformations as well as cause other problems. In many cases, the firstwarning of the existence of a significant scale deposit may be a declinein well performance. In these instances, scale removal techniques maybecome necessary. As a result, a potentially substantial cost includingdowntime is required lost to effect repair as a result of scaling.

Scale inhibiting materials are often added directly to a fluid to betreated or applied to oil bearing rock formations by means of “squeezetreatment”. Squeeze treatment is a treatment used to control or preventscale deposition in a rock formation. In the squeeze application, thescale inhibitor is pumped into a water-producing zone and attaches tothe formation by chemical adsorption or by temperature-activatedprecipitation. When the well is put back into production, the scaleinhibitor leaches out of the formation rock to treat the fluid. Somechemicals typically used in scale-inhibitor squeeze applications includephosphonated carboxylic acids or polymers.

Scale formation is only controlled if the scale inhibitor polymer ispresent at a treatment level equal to or above the product's definedminimum inhibitor concentration. When the scale inhibitor is below theminimum inhibitor concentration such as may occur during use, adsorptionor degradation, additional amounts are then needed. For example, oncethe well is subjected to the squeeze application and the well is againoperational, the concentration of the scale inhibitor in the producedfluids will diminish over time until such time that the scale inhibitoris at about or below the minimum inhibitor concentration. However, it isdifficult to determine when more scale inhibitor is needed and in whichconduit or well it is needed. To address this problem, scale inhibitorsare often tagged or labeled so that the presence or absence of the scaleinhibitor can be readily detected. Prior art scale inhibitors aregenerally tagged by introduction of specific atoms such as phosphorous,boron, and the like such that the concentration can be readily detectedby inductively coupled plasma (ICP) analysis for the tagged atom.Alternatively, the scale inhibitor can be tagged fluorescent moieties.However these compounds are generally limited to structures that includeone or more conjugated six member benzene rings that fluoresce at about292 nm.

While the prior art fluorescent tagged scale inhibitors are suitable fortheir intended, there is a need in the art for additional fluorescentmoieties that emit at different wavelengths, which would be beneficialin multi-tagged systems where multiple scale inhibitors with tags havingdifferent fluorescent wavelengths are utilized. Examples of such asystem would be when more than one wells are drilled and the oil iscollected from one central location. The multi-tagged system would allowthe operators to know which specific wall requires more antiscalent,simply by looking at what frequency is missing.

BRIEF DESCRIPTION OF THE INVENTION

Disclosed herein are scale inhibition compositions and methods ofinhibiting scale. In one embodiment, the scale inhibition (co)polymerincludes at least one scale inhibiting polymerized monomer selected fromthe group consisting of acrylic acid; vinyl sulfonic acid or vinylsulfonate salts; vinyl phosphoric acid or vinyl phosphonate salts;vinylidene diphosphonic acid or salts thereof; methacrylic acid; vinylacetate; vinyl alcohol; vinyl chloride; unsaturated mono- ordi-carboxylic acids or anhydrides; vinyl chloride; styrene-p-sulfonicacid, or styrene sulfonates salts; acrylamido-2-methylpropanesulfonicacid (AMPS); hydroxyphosphonoacetic acid (HPA); hypophosphorus acidsacrylamides, propargyl alcohol having formula HC≡C—CH₂—OH;butyr-1,4-diol, and mixtures thereof; and a 1-vinyl imidazolepolymerized monomer of the formula:

wherein R, R₁ and R₂ are the same or different, and each R, R₁, and R₂is selected from the group consisting of H, an alkyl group, an aromaticgroup, a phosphate group, a nitrate group, and a sulfate group.

In another embodiment, a process for determining a concentration of ascale inhibiting (co)polymer for inhibiting scale formation comprisesintroducing an effective amount of the scale inhibition (co)polymer toan aqueous medium to inhibit calcium carbonate, calcium sulfate, bariumsulfate, and/or calcium phosphate scale formation, wherein the scaleinhibition (co)polymer includes at least one scale inhibiting moiety andan imidazole moiety formed by polymerizing at least one scale inhibitingmonomer and a 1-vinylimidazole monomer; measuring a fluorescence signalcorresponding to the imidazole moiety; and determining a concentrationof the scale inhibiting (co)polymer based on the fluorescence signal.

In another embodiment, a water treatment solution for inhibiting scaleformation comprises a scale inhibiting (co)polymer including animidazole moiety, comprising at least one scale inhibiting polymerizedmonomer selected from the group consisting of acrylic acid; vinylsulfonic acid or vinyl sulfonate salts; vinyl phosphoric acid or vinylphosphonate salts; vinylidene diphosphonic acid or salts thereof;methacrylic acid; vinyl acetate; vinyl alcohol; vinyl chloride;unsaturated mono- or di-carboxylic acids or anhydrides; vinyl chloride;styrene-p-sulfonic acid, or styrene sulfonates salts;acrylamido-2-methylpropanesulfonic acid (AMPS); hydroxyphosphonoaceticacid (HPA); hypophosphorus acids acrylamides, propargyl alcohol havingformula HC≡C—CH₂—OH; butyr-1,4-diol, and mixtures thereof; and a 1-vinylimidazole polymerized monomer of the formula:

wherein R, R₁ and R₂ are the same or different, and each R, R₁, and R₂is selected from the group consisting of H, an alkyl, an aromatic,phosphate, nitrate, and sulfate; and a solvent.

The disclosure may be understood more readily by reference to thefollowing detailed description of the various features of the disclosureand the examples included therein.

DETAILED DESCRIPTION

The present disclosure is generally directed to scale inhibitingcompositions and methods for inhibiting scale formation. Thecompositions generally include a scale inhibiting (co)polymer includingan imidazole moiety, wherein the fluorescence of the imidazole moietycan be used to determine the concentration of the scale inhibiting(co)polymer. As used herein, the term (co)polymer is generally definedas materials that are produced by polymerization of more than one typeof monomer (including 2, 3, 4, or more different monomers) withoutrestriction on the number of monomer units that are incorporated intothe product provided that at least one of the monomers is a scaleinhibiting moiety and at least one of the monomers is an imidazolemoiety. The imidazole moiety, without substitution, fluoresces at about424 nanometers (nm), thereby providing the scale inhibitor (co)polymerwith a means for monitoring the concentration of the scale inhibitor(co)polymer at a wavelength different from prior art's monitoring ofspecific atoms or scale inhibiting polymers tagged with one or morearomatic benzene rings. Because of the difference in wavelength, thescale inhibiting (co)polymer including the imidazole moiety can be usedin multi-tagged systems.

Scale inhibiting (co)polymers including the imidazole moiety areobtainable by free radical polymerization of one or more scaleinhibiting monomers with a 1-vinylimidiazole monomer of the formula (1):

wherein R, R₁ and R₂ are the same or different, and each can be H, analkyl, or an aromatic group. In some embodiments, R, R₁ and R₂ mayinclude inorganic functionality, such as: phosphates, nitrates, orsulfates. As will be appreciated by those skilled in the art, thesubstituent can be used to shift the maximum wavelength fluorescenceemission. When R, R₁ and R₂ are H, the imidazole moiety fluoresces atabout 424 nm.

Suitable scale inhibiting monomers include, without limitation, acrylicacid; vinyl sulfonic acid or vinyl sulfonate salts; vinyl phosphoricacid or vinyl phosphonate salts; vinylidene diphosphonic acid or saltsthereof; methacrylic acid; vinyl acetate; vinyl alcohol; vinyl chloride;unsaturated mono- or di-carboxylic acids or anhydrides, such as maleicanhydride, maleic acid, fumaric acid, itaconic acid, aconitic acid,mesaconic acid, citraconic acid, crotonic acid isocrotonic acid, angelicacid, tiglic acid; vinyl chloride; styrene-p-sulfonic acid, or styrenesulfonates salts; allyl sulfonate salts;acrylamido-2-methylpropanesulfonic acid (AMPS); hydroxyphosphonoaceticacid (HPA); hypophosphorus acids such as H₃PO₃, giving units of formula—PO(OH)—; acrylamides, propargyl alcohol having formula H≡C—CH₂—OH;butyr-1,4-diol, hydroxyethylmethacrylate (HEMA), hydroxyethylacrylate(HEA) and mixtures thereof.

While the above-mentioned scale inhibiting monomers are usuallycomprised in the polymer backbone, other monomers and/or other groupsmay also be included. For example, the other groups can result from apolymerization initiator or can be end-capping groups.

The polymerization of the monomers can be carried out in the presence ofpolymerization initiators including, without limitation, ammoniumpersulfate, sodium persulfate, Vazo initiators, AIBN, organic orinorganic peroxides, cerium ammonium nitrate, perchlorates, and thelike. The polymerization initiators are generally in an amount of about0.01 to about 3 weight percent based on the total weight of themonomers.

Any polymerization method can be used to prepare the polymers.Free-radical polymerization methods are generally preferred. Suitablemethods include aqueous bulk/dispersion polymerization, solutionpolymerization, or emulsion polymerization. Preferably, thepolymerization process is solution polymerization, wherein water ischarged to a reaction vessel fitted with a mechanical stirrer and watercondenser and heated to a temperature within a range of 45 to 110° C.One or more polymerization initiators may be added or these may be fedin later. The 1-vinylimidazole monomer may also be added to this charge,added to the polymer feed or fed in separately. A monomer feed(s),soluble initiator feed and optionally a chain transfer reagent feed areadded to the vessel over a period of time.

The imidazole moiety in the scale inhibiting (co)polymer is generally ata molar amount of less than 20% by monomer. In other embodiments, theimidazole moiety is at a weight percent of total monomer of 10% to 20%;and in still other embodiments, the imidazole moiety is at a weightpercent of 2 to 10%.

The scale inhibiting (co)polymer has a weight average molecular weightof 1200 to 15000 Daltons; in other embodiments, the weight averagemolecular weight is 4000 to 10000 Daltons, and in still otherembodiments, the weight average molecular weight is 1500 to 3000Daltons.

The scale inhibitor polymer including the imidazole moiety can bedetected by fluorometry, for example using a fixed wavelengthfluorometer.

The scale inhibiting (co)polymer can be used as a scale inhibitor in anyindustrial water system where a scale inhibitor is needed. Suitableindustrial water systems, include, without limitation, cooling towerwater systems (including open recirculating, closed and once-throughsystems); petroleum wells, downhole formations, geothermal wells andother oil field applications; boilers and boiler water systems; mineralprocess waters including mineral washing, flotation and benefaction;paper mill digesters, washers, bleach plants and white water systems;black liquor evaporators in the pulp industry; gas scrubbers and airwashers; continuous casting processes in the metallurgical industry; airconditioning and refrigeration systems; industrial and petroleum processwater; indirect contact cooling and heating water, such aspasteurization water; water reclamation and purification systems;membrane filtration water systems; food processing streams (meat,vegetable, sugar beets, sugar cane, grain, poultry, fruit and soybean);and waste treatment systems as well as in clarifiers, liquid-solidapplications, municipal sewage treatment and industrial or municipalwater systems.

This effective amount of the scale inhibitor (co)polymer will generallyvary depending on the particular system to be treated and scaleinhibiting moieties in the scale inhibiting (co)polymer and will beinfluenced by factors such as the area subject to deposition, pH,temperature, water quantity, and the respective concentration in thewater of the potential scale and deposit forming species. For the mostpart, the treatment solution according to the present disclosure will beeffective when the scale (co)polymer used at levels less than 35 partsper million. In some embodiments, the composition is effective atconcentrations of about 4 to about 7 parts per million and in stillother embodiments; the effective concentration is about 0.5 to about 3parts per million. The composition can be added directly into thedesired aqueous system to be treated in a fixed quantity or can beprovided as an aqueous solution and added continuously or intermittentlyto the aqueous system as can be desired for some applications.

By way of example, the compositions of the scale inhibitor (co)polymercan be suitably used in oilfield injection and production waters,including topside, downhole and rock formation squeeze applications atthe well site. In oilfield injection and production waters, scaleformation can constrict injection lines, flow lines, and tubing strings.Without being limited by theory, the above noted composition functionsby modifying the crystal growth of nucleating scale particles andinterrupting and delaying crystal growth. They also sequester metalions, making them unavailable for ion pairing with anions and hencepreventing precipitation of insoluble scale.

In one embodiment, the scale inhibiting (co)polymer composition isutilized in a squeeze application. The scale inhibiting (co)polymer isdiluted in a suitable carrier solvent (usually brine) and propagated outto an optimized radial distance into the oil producing formation, whereit is retained and then released slowly back into the aqueous phaseduring normal well production. In one embodiment, the squeeze processgenerally includes applying a dilute solution of the scale inhibiting(co)polymer with surfactant (0.1%) to clean and cool the near wellbore.Once cleaned, a high concentration solution of the scale inhibiting(co)polymer at between 5 and 20% is introduced, followed by a lowconcentration solution of the scale inhibiting (co)polymer. Thesolutions are left in contact with the reservoir for a period of timeeffective to allow for adsorption equilibration, after which the well isreturned to production. Adhesion to the formation allows the scaleinhibiting (co)polymer to remain within the near-wellbore area withoutbeing pumped up in the oil/water emulsion. Although squeeze applicationof the chemical is the most common method of treating downhole scale,the product could also be applied by other techniques commonly usedoffshore, which include gas-lift injection, downhole annulus injection,encapsulation or soluble matrix techniques, sub-sea wellhead injectionvia umbilical or indeed secondary topside treatments to enhanceinhibitor performance as process conditions vary scaling tendency.

Prior to application of the product, experiments can be conducted in alaboratory to determine an effective minimum inhibitor concentration(MIC) which just inhibits inorganic scale formation under simulatedproduction conditions. The ability of the operator to quickly andaccurately determine the amount of scale inhibitor in the producedfluids and compare this to the MIC values generated allows him to decidewhen it is most suitable to retreat the reservoir or increase thetopside addition rate to ensure that no damage occurs to his reservoiror equipment due to inorganic scale deposition.

The following examples are presented for illustrative purposes only, andare not intended to be limiting.

EXAMPLES

In the following examples, maleic anhydride, vinyl imidazole andethylenediamine tetraacetic acid were obtained from Sigma-Aldrich Co.Sodium allyl sulfonate was obtained from McGean Rohco, Inc Ammoniumpersulfate was obtained from Kemira Chemicals, Inc.

Example 1

In this example, sodium allyl sulfonate was tagged with 10% vinylimidazole (SASMAC/10% IM). Sodium allyl sulfonate (340.10 g, maleicanhydride (27.84 g) vinyl imidazole (15.10 g) and EDTA (4.72 g) werecharged to a reactor. The reactor was equipped with a Teflon coatedthermocouple, a nitrogen inlet, one feeding tube inlet, a glass andTeflon stir shaft, and a heating mantle. The reaction was carried out at90° C. with constant flow of nitrogen and low agitation. One feedingtank containing ammonium persulfate (6.96 g) in water (32.0 g) wasprepared and fed into the reactor over a two hour period. Maleicanhydride (27.84 g) was fed manually over the 2 hour initiator period.Once the initiator and maleic anhydride feeds were complete, thereaction was held at 90° C. for one hour before cooling to roomtemperature.

Example 2

In this example, sodium allyl sulfonate was tagged with 5% vinylimidazole (SASMAC/5% IM). Sodium allyl sulfonate (340.10 g, maleicanhydride (27.84 g) vinyl imidazole (7.55 g) and EDTA (4.72 g) werecharged to a reactor and reacted in accordance with Example 1. Once theinitiator and maleic anhydride feeds were complete, the feed line wasflushed with water (5 g) and the reaction was held at 90° C. for onehour before cooling to room temperature.

Example 3

In this example, polyacrylate was tagged with varying amounts of vinylimidazole (AA/IM) having a molecular weight of about 2,000 Daltons. Thereactor was equipped with a Teflon coated thermocouple, a nitrogeninlet, three feeding tube inlets, a stainless steel stir shaft, and aheating mantle. Water (190.7 g) was charged to the reactor and heated to85° C. The reaction was carried out at 85° C. with constant flow ofnitrogen and low agitation.

Acrylic Acid/2% Imidazole (AA/2% IM)

Feed Tank 1: glacial acrylic acid (279.7 g), vinyl imidazole (5.71 g)Feed Tank 2: water (89.5 g), sodium persulfate (12.7 g)Feed Tank 3: water (44.8 g), sodium bisulfite (161.0 g)

Acrylic Acid/5% Imidazole (AA/5% IM)

Feed Tank 1: glacial acrylic acid (271.2 g), vinyl imidazole (14.2 g)Feed Tank 2: water (89.5 g), sodium persulfate (12.7 g)Feed Tank 3: water (44.8 g), sodium bisulfite (161.0 g)

Acrylic Acid/10% Imidazole (AA/10% IM)

Feed Tank 1: glacial acrylic acid (256.9 g), vinyl imidazole (28.5 g)Feed Tank 2: water (89.5 g), sodium persulfate (12.7 g)Feed Tank 3: water (44.8 g), sodium bisulfite (161.0 g)

Acrylic Acid/20% Imidazole (AA/20% IM)

Feed Tank 1: glacial acrylic acid (228.5 g), vinyl imidazole (57.0 g)Feed Tank 2: water (89.5 g), sodium persulfate (12.7 g)Feed Tank 3: water (44.8 g), sodium bisulfite (161.0 g)

Each reagent mixture was fed into a reactor over a four hour period.Once the feeds were complete, the feed lines were flushed with water(10.0 g) and the reaction mixture held at 85° C. for one hour beforecooling to room temperature. The pH was adjusted to 6.5 using 50% sodiumhydroxide.

Table 1 provides the flurescence intensities at 424 nm at differentlevels of concentration for the various (co)polymers.

Intensity Intensity Intensity Polymer Mw IM (%) CaCO₃ BaSO₄ (0.85 ppm)(3.4 ppm) (34 ppm) SASMAC/IM 10 — 3.6 14.39 18.905 133.18 SASMAC/IM 5 —3.6  9.81 25.048 99.319 AA/IM 3328 2 2.4 5 — 7.007 20.86 AA/IM 3279 52.7 5 — 10.199 32.92 AA/IM 2902 10 2.4 5 — 19.92 33.96 AA/IM 3003 20 2.45 — 17.54 64.76 AA/IM 1924 2 2   6.4 — 6.24 15.68 AA/IM 1800 5 Not 5.5 —7.85 26.51 complete AA/IM 1436 10 2 ppm = 91% 5.5 — 13.764 26.25 AA/IM1469 20 Not Not — 6.54 58.76 complete complete

The data shows that the vinylimidazole moiety in the scale inhibiting(co)polymers provided an effective amount of fluorescence at about 424nm for the different amounts of scale inhibitor present. Thefluorescence emission signal at about 424 nm makes the scale inhibitor(co)polymer suitable for use in multi-tagged systems. For example, thescale inhibiting (co)polymer with the imidazole moiety can be used witha sodium styrene sulfonate inhibitor, which by itself fluoresces at 292nm

The imidazole (co)polymers provide scale inhibition properties providethe ability to monitor scale inhibitor levels during the oil or miningapplications, among others.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and can include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

1-6. (canceled)
 7. A process for determining a concentration of a scaleinhibiting (co)polymer for inhibiting scale formation, the processcomprising: introducing an effective amount of the scale inhibition(co)polymer to an aqueous medium to inhibit calcium carbonate, calciumsulfate, barium sulfate, and/or calcium phosphate scale formation,wherein the scale inhibition (co)polymer includes at least one scaleinhibiting moiety and an imidazole moiety formed by polymerizing atleast one scale inhibiting monomer and a 1-vinylimidazole monomer;measuring a fluorescence signal corresponding to the imidazole moiety;and determining a concentration of the scale inhibiting (co)polymerbased on the fluorescence signal.
 8. The process of claim 7, wherein theat least one scale inhibiting monomer is selected from the groupconsisting of acrylic acid; vinyl sulfonic acid or vinyl sulfonatesalts; vinyl phosphoric acid or vinyl phosphonate salts; vinylidenediphosphonic acid or salts thereof; methacrylic acid; vinyl acetate;vinyl alcohol; vinyl chloride; unsaturated mono- or di-carboxylic acidsor anhydrides; vinyl chloride; styrene-p-sulfonic acid, or styrenesulfonates salts; acrylamido-2-methylpropanesulfonic acid (AMPS);hydroxyphosphonoacetic acid (HPA); hypophosphorus acids acrylamides,propargyl alcohol having formula H≡C—CH₂—OH; butyr-1,4-diol, andmixtures thereof.
 9. The process of claim 7, wherein the at least onescale inhibiting monomer is the acrylic acid.
 10. The process of claim7, wherein the at least one scale inhibiting monomer is a sodium allylsulfonate monomer and the acrylic acid monomer.
 11. The process of claim7, wherein the imidazole moiety is at a wt % of total monomer of 2 to20, wt % of the scale inhibiting (co)polymer.
 12. The process of claim7, wherein the average molecular weight is 2000 to 3500 Daltons.
 13. Theprocess of claim 7, wherein the effective amount is less than 35 partsper million.
 14. The process of claim 7, wherein the effective amount is0.5 to 35 parts per million.
 15. The process of claim 7, wherein theprocess is a squeeze application.
 16. A water treatment solution forinhibiting scale formation, comprising: a scale inhibiting (co)polymerincluding an imidazole moiety, comprising at least one scale inhibitingpolymerized monomer selected from the group consisting of acrylic acid;vinyl sulfonic acid or vinyl sulfonate salts; vinyl phosphoric acid orvinyl phosphonate salts; vinylidene diphosphonic acid or salts thereof;methacrylic acid; vinyl acetate; vinyl alcohol; vinyl chloride;unsaturated mono- or di-carboxylic acids or anhydrides; vinyl chloride;styrene-p-sulfonic acid, or styrene sulfonates salts;acrylamido-2-methylpropanesulfonic acid (AMPS); hydroxyphosphonoaceticacid (HPA); hypophosphorus acids acrylamides, propargyl alcohol havingformula H≡C—CH₂—OH; butyr-1,4-diol, and mixtures thereof; and a 1-vinylimidazole polymerized monomer of the formula:

wherein R, R₁ and R₂ are the same or different, and each R, R₁, and R₂is selected from the group consisting of H, an alkyl, an aromatic,phosphate, nitrate, and sulfate; and a solvent.
 17. The water treatmentsolution of claim 16, wherein the solvent is brine or water.
 18. Thewater treatment solution of claim 16, wherein the scale inhibiting(co)polymer is in an amount of less than 35 parts per million.
 19. Thewater treatment solution of claim 16, wherein the scale inhibiting(co)polymer is in an amount of less than 0.5 to 35 parts per million.20. The water treatment solution of claim 16, wherein the imidazolemoiety is at a weight percent of monomer of 2 to 20% of the scaleinhibiting (co)polymer.
 21. The water treatment solution of claim 16,wherein the average molecular weight is 1200 to 2000 Daltons.
 22. Thewater treatment solution of claim 16, further comprising at least oneadditional tagged scale inhibiting material having an fluorescenceemission signal different from the scale inhibiting (co)polymer.